This application claims priority from Korean Patent Application No. 2003-49547, filed on Jul. 19, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to alloy type semiconductor nanocrystals and a method of preparing the same. More particularly, the present invention relates to alloy type Group 12-16 compound semiconductor nanocrystals with a high luminance efficiency in a visible light band and a wet preparation method for the semiconductor nanocrystals.
2. Description of the Related Art
When compound semiconductor materials are made into nanometer-sized crystals, a quantum confinement effect occurs at the regions of the crystals smaller than the bulk exciton Bohr radius. Such a quantum confinement effect results in a change in bandgap energy which is intrinsic characteristics of semiconductor materials. In a case where visible light-emitting compound semiconductor materials are made into nanocrystals, a bandgap energy commences to increase when the size of the nanocrystals reaches below a specific level. As a result, the smaller sized nanocrystals exhibit a blue shift of a luminance band. Based on such size-dependent optical characteristics of quantum dot materials, adjustments of intrinsic properties, structures, shapes, and sizes of the quantum dot materials enable a change in an energy bandgap, which allows formation of various energy levels.
Studies have been done on a quantum dot growth technology as the most important technology among next generation semiconductor device development technologies. In particular, metal organic chemical deposition (MOCVD) and molecular beam epitaxy (MBE) which are conventional vapor phase deposition processes are promising technologies which allow a control of a semiconductor thin film to a single atomic layer level and a controllable growth of quantum dots. Even though quantum dots grown mainly by lattice mismatch using a vapor phase method have good crystallinity, the vapor phase method has a fatal defect in controlling the size, uniformity, and density of the quantum dots, which renders fabrication of commercially available semiconductor devices difficult.
In view of these problems, there was an attempt to grow quantum dots using a chemical wet method. According to the chemical wet method, quantum dot crystal precursor materials grow into quantum dot crystals in a coordinating organic solvent. The organic solvent spontaneously coordinates with the surfaces of the quantum dot crystals. At this time, the organic solvent serves as a dispersing agent to control the size of the quantum dot crystals to a nanometer level.
U.S. Pat. No. 6,225,198 B1 discloses a method for forming Group 12-16 compound semiconductor quantum dots. According to the method disclosed in this patent, a Group 12 metal (Zn, Cd, or Hg) containing material which is a Group 12 precursor is dissolved in a first dispersion and a Group 16 element (S, Se, or Te) containing material which is a Group 16 precursor is dissolved in a second dispersion. A solvent capable of dissolving the two precursors is added to a mixture of the two dispersions and maintained at a temperature sufficient to promote the growth of Group 12-16 compound semiconductor crystals. When the sizes of the compound semiconductor crystals reach a desired level, the crystals are separated. However, this method has a restriction on the solvent used, i.e., tri-octyl phosphoric acid (referred to as TOPO, hereinafter). TOPO is commercially available only as technical grade (about 90% pure) TOPO. Reportedly, it is difficult to form reproducible and uniform quantum structures in a solvent containing a large amount of impurities. It is also reported that impurities in the technical grade TOPO serves as an uncontrollable reaction parameter that adversely affects the reaction. In this regard, substitution of pure (99%) TOPO for the technical grade TOPO can be considered. In this case, however, a change in a binding property of TOPO may occur, which renders the growth of desired crystals difficult.
U.S. Pat. No. 6,306,736 discloses a method for forming Group 3-5 compound semiconductor quantum dots. Here, the above-described method for forming Group 12-16 compound semiconductor quantum dots is used as it is.
U.S. Pat. No. 6,322,901B1 discloses high-luminescent core-shell quantum dot materials and U.S. Pat. No. 6,207,229 discloses a method for preparing core-shell quantum dot materials. It is reported that the core-shell compound semiconductor quantum dots thus formed exhibit an increase in luminance efficiency by 30 to 50%.